Whether you agree or disagree, President Obama’s surprising decision to ask Congress to authorize the use of force against Syria was last weekend’s question. Now that the Senate has launched a vigorous debate, the vital question is whether the United States will respond to the Syrian government’s use of chemical weapons against innocent civilians.

From a foreign policy perspective, the decision isn’t even close — the United States must act by attacking President Bashar Assad’s air force, artillery, and command and control assets within Syria. The goal is to intimidate him, degrade his military capacity, and deter him from ever using these weapons again. There are risks, to be sure, in any use of force. But this will not be another Iraq — the United States will not put ground troops into Syria. And the risks are even greater if we do nothing.

Syria isn’t a sideshow. Given its location in the heart of the Arab world, next to US friends Israel, Jordan, Turkey, Lebanon, and Iraq, what happens in Syria matters deeply to the United States. We have another important interest. That is to back up the commitment President Obama made more than a year ago — that if Assad used chemical weapons, there would be a price to pay. He has done so twice. We now need to act.

Here are five reasons why Congress should authorize the use of force.

First, the global ban on chemical weapons needs to be enforced. The names of some of the leaders who have ordered their use are telling — Adolf Hitler, Saddam Hussein, and now Assad. Chemical weapons have been banned since just after the First World War. Absent an effective United Nations, the only sheriff that can enforce this vital international legal prohibition is the United States. If we don’t insist on a zero tolerance standard, the chemical weapons genie will be out of the bottle forever, innocent Syrian civilians might be attacked again, and other dictators may be emboldened to use these agents with impunity.

Second, Iran is watching. The greater challenge for the next 12 months is whether we will negotiate the end of the Iranian nuclear program or strike Iran to stop it. The Iranian government will gauge how Obama deals with Assad. American resolve on Syria makes it more likely Iran will negotiate seriously on nukes rather than risk a US strike down the line. A cardinal rule in international politics is that force often aids diplomacy. We may have a much better chance of negotiating a deal with Iran on nukes if we are tough now on Syria. And the air strikes may also give Obama leverage over Assad to insist on negotiations for a ceasefire. In this way, the United States would combine force and diplomacy to try to end the fighting.

Third, striking Assad may also weaken the Iran/Hezbollah/Russia axis that forms Syria’s international lifeline. This axis is a strategic challenge to every important American interest in the region. If we can weaken its position in the region, we should do so. This is why Senator John McCain is right to argue for a strike strong enough to make a strategic difference.

Fourth, America’s credibility is at stake. Opponents of a strike question whether America’s reputation will really suffer if we fail to act. But nations, like individuals, are judged by whether they honor their promises. Credibility is a tangible commodity in international politics. It takes decades to acquire a good reputation but only a short time to see it evaporate. McCain is right again to warn that congressional failure to authorize force in Syria would be a “catastrophe” for America’s credibility.

Fifth, there is a larger issue at stake: Syria is also a test of our global leadership. The United States is, in the words of Princeton’s John Ikenberry, the “system operator” of the international order. Without American energy and attention, the world order cannot function well.

This is not a message the growing number of neo-isolationists in both political parties in Congress want to hear. But it is fundamental to our wish to live in a world that has rules, order, civility, and justice. Without constant, effective, and smart US leadership, the global order will break down. Ronald Reagan, the two Bushes, and Bill Clinton all understood this. President Obama does too. The isolationists would have us retreat from leadership — a recipe for failure in the globalized 21st century.

This is one of those moments. The United States has to act in Syria.

Nicholas Burns is a professor of the practice of diplomacy and international politics at Harvard’s Kennedy School of Government. Follow him on Twitter @rnicholasburns. He wrote this column for the Boston Globe; an earlier version of it appeared in Global Post.

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